Industrial wastewaters commonly include a variety of contaminants which require treatment (i.e., removal) even before the wastewater can be discharged from the plant site. The nature of the wastewater contaminants is in large part dependent on the commercial processes practiced in the plant. Accordingly, there is great variety in the nature of wastewater contaminant problems. Moreover, the matrix (i.e., makeup) of wastewater even at a given commercial site will usually vary, sometimes dramatically, with changes in production or the like.
Particular industries, for example such as those relating to metal plating, metal finishing, or circuit board manufacturing activities, generate wastewater with heavy metals (e.g., copper, nickel, etc.) and other metals in solution with such wastewater. The commercial activities themselves may inherently generate heavy metals which are chelated and/or complexed for purposes of the commercial activity (e.g., metal plating) itself. Chelating and/or complexing tends to cause such metals to remain in solution, and thus require special attention for their removal.
During the typical course of plant activity, heavy metal concentration in the wastewater is highly variable. While concentration variations can in general be expected, monitoring of and reacting to specific variations is problematic. Concentrations of heavy metals may typically vary from a few parts per million to several hundred parts per million, even in a very short time, such as a matter of minutes.
Not only do concentration levels vary drastically, but also extreme variations can be experienced with respect to the matrix (both in identity and nature, e.g., chelated versus non-chelated) of heavy metals present.
In general, it is known to add (i.e., feed) various precipitating agents to wastewater to precipitate such heavy metals for their removal from the water. The amount of such precipitating agents required (i.e. consumed) in the course of precipitating such heavy metals of course depends on the degree of presence of such heavy metals in solution with the wastewater. Since effective real time monitoring of heavy metal concentration levels has heretofore generally proven difficult, such treatment chemical feeding (i.e. the feed rate of precipitating agents) is typically set at a compromise level, such as for precipitating the maximum expected concentration of heavy metals. Such a compromise setting creates an excess amount of sludge, which sludge may often be classified as a hazardous waste. Moreover, since the cost of the treatment chemicals is not insignificant, wasteful overfeeding thereof is costly.
Operators have been known to attempt periodic checks to manually detect the level of metals entering the wastewater (i.e., assess the expected concentrations), and adjust the chemical feed rate accordingly. However, such a manual adjustment merely alters the set feed rate in accordance with periodic reassessments of the anticipated maximum concentration, and does nothing to eliminate excess sludge production and excessive and costly chemical usage caused by differences between actual concentration levels and the anticipated maximums thereof. Moreover, short-term spikes can still occur, meaning that inadequately treated wastewater can be nonetheless discharged. Such occurrences are particularly problematic where applicable laws regulate the permissible discharge concentration levels, such as to certain fractional parts per million or certain parts per million.
In some industrial settings, anticipation of heavy metal concentrations in the wastewater may be relatively less “predictable”. For example, a totally unexpected occurrence of heavy metals in the wastewater can go unchecked, thereby causing the plant to exceed permissible discharge levels. For example, maintenance personnel might empty mop buckets or the like containing chelated heavy metals picked up from the floor of the facility, which could cause a heavy metal concentration spike in the wastewater at a time whenever commercial activity in the plant is nil, and precipitating agent feed pumps may be switched off. The plant is nonetheless responsible for its wastewater discharge, though no effective continuous monitoring systems for preventing such undesirable discharges may be available.
It is generally known that certain metals in solution in wastewater may be precipitated therefrom by controlling the pH level of the wastewater. For example, non-chelated and non-complexed metals in particular may be in various degrees precipitated in such manner. Automatic controllers are generally available which function to probe the wastewater for its pH level, and automatically pump treatment chemicals accordingly to the wastewater so as to adjust its pH level within an established deviation from a pre-selected set point. One example of such a controller is the Model 5 proportional pH pump controller, made by Chem-Tech International, Inc., of 92 Bolt Street, Lowell, Mass., 01853. While such a controller may be effective for metals which may be precipitated through such pH inducement, heavy metals which are chelated and/or complexed generally will not be precipitated with such pH level control. Thus, the monitoring and treatment problems noted above persist, and may be compounded where a changing mix of chelated and non-chelated metals is presented for treatment.
Another aspect of wastewater treatment problems where both such types of metals are in solution (i.e., which can and can not be practically precipitated through pH inducement) is that use of a precipitating agent can precipitate both such types of metals. However, unnecessary sludge production is caused by precipitating metals in such a manner which could have otherwise been precipitated through pH level control (as generally discussed above). Again, the amount of precipitating agent consumption is also a factor.
In addition to the availability of known pH level control generally outlined above, at least one other generally known method, involving a so-called oxidation reduction potential probe, attempts to address precipitating agent usage. Such a probe is typically used to detect the presence of excess (i.e. un-consumed) precipitating agent at a phase of a wastewater treatment program after all the metal is removed. One particular limitation of such a system is that it cannot distinguish between, for example, chelated and non-chelated metals, and must therefore feed precipitating agent until there is an excess of such agent present in the water. Feed control feedback also is derived from detected excess agent, not from information relative remaining metal in solution to be precipitated. Thus, there is no effective prevention of excess sludge generation or wasteful chemical usage.
Another limitation of a waste treatment system utilizing an oxidation reduction potential (ORP) probe is that the probe operation involves an electrical measurement which is affected by changes in the pH level of the wastewater, the amount of total dissolved solids therein, the oxidizer concentration, and the amount of chelated metal in the wastewater. Thus, an ORP probe system is inherently ineffective for use in providing close control of the feeding of chemical treatment solutions into wastewater treatment systems.
For example, U.S. Pat. Nos. 5,045,213; 4,999,116; and 4,923,599, all to the present common assignee, all incorporated herein by reference, are directed to wastewater treatment for the removal of heavy metals, which is optimized by continuously removing and filtering a sample flow of treated wastewater subject to pH level control to determine the presence of remaining metals in solution to be precipitated
Thus, chemical precipitation is the most common technology used to remove dissolved (ionic) metals from solutions, such as process wastewaters containing toxic metals. The ionic metals are converted to an insoluble form (particle) by the chemical reaction between the soluble metal compounds and the precipitating reagent. The particles formed by this reaction are removed from solution by settling and/or filtration. The effectiveness of a chemical precipitation process is dependent on several factors, including the type and concentration of ionic metals present in solution, the precipitant used, the reaction conditions (especially the pH of the solution), and the presence of other constituents that may inhibit the precipitation reaction.
Further, in wastewater treatment operations, the processes of coagulation and flocculation are employed to separate suspended solids from water. Although the terms coagulation and flocculation are often used interchangeably, or the single term “flocculation” is used to describe both; they are, in fact, two distinct processes.
Finely dispersed solids (colloids) suspended in wastewaters are stabilized by negative electric charges on their surfaces, causing them to repel each other. Since this prevents these charged particles from colliding to form larger masses, called flocs, they do not settle. To assist in the removal of colloidal particles from suspension, chemical coagulation and flocculation are required. These processes, usually done in sequence, are a combination of physical and chemical procedures. Chemicals are mixed with wastewater to promote the aggregation of the suspended solids into particles large enough to settle or be removed.
Coagulation is the destabilization of colloids by neutralizing the forces that keep them apart. Cationic coagulants provide positive electric charges to reduce the negative charge (zeta potential) of the colloids. As a result, the particles collide to form larger particles (flocs). Rapid mixing is required to disperse the coagulant throughout the liquid. Care must be taken not to overdose the coagulants as this can cause a complete charge reversal and restabilize the colloid complex.
Flocculation is the action of polymers to form bridges between the flocs and bind the particles into large agglomerates or clumps. Bridging occurs when segments of the polymer chain adsorb on different particles and help particles aggregate. An anionic flocculant will react against a positively charged suspension, adsorbing on the particles and causing destabilization either by bridging or charge neutralization. In this process it is essential that the flocculating agent be added by slow and gentle mixing to allow for contact between the small flocs and to agglomerate them into larger particles. The newly formed agglomerated particles are quite fragile and can be broken apart by shear forces during mixing. Care must also be taken to not overdose the polymer as doing so will cause settling/clarification problems. Anionic polymers themselves are lighter than water. As a result, increasing the dosage will increase the tendency of the floc to float and not settle.
Once suspended particles are flocculated into larger particles, they can usually be removed from the liquid by sedimentation, provided that a sufficient density difference exists between the suspended matter and the liquid. Such particles can also be removed or separated by media filtration, straining or floatation. When a filtering process is used, the addition of a flocculant may not be required since the particles formed by the coagulation reaction may be of sufficient size to allow removal. The flocculation reaction not only increases the size of the floc particles to settle them faster, but also affects the physical nature of the floc, making these particles less gelatinous and thereby easier to dewater.
Thus, representative of typical prior art patents, U.S. Pat. No. 5,328,599 is directed to a wastewater treatment system and method for chemical precipitation and removal of metals from wastewater in a continuous or batch treatment process which includes an ion-selective electrode and a reference electrode disposed in a precipitation tank for measuring an electrochemical potential therebetween in a predetermined range. A controller unit is provided which is responsive to the electrochemical potential in the predetermined range and is connected to a precipitant feed unit for automatically controlling the chemical precipitant fed into the precipitation unit.
U.S. Pat. No. 5,645,799 is directed to an apparatus for optimizing the dosage of a chemical wastewater treatment agent employing a fluorescent tracer. The apparatus includes a series of components that sample the waste stream, process the sample for analysis, analyze the sample, record the data in a historical database, and, based upon the analysis as compared to historical data, adjust the chemical feed system to optimize the chemical wastewater treatment agent according to a programmed optimization logic.
Photography has been employed to a limited extent in wastewater treatment processes. For example, U.S. Pat. No. 4,654,139 is directed to a flocculating system in which the speed at which water in a flocculating tank is mixed varies with the size of flocs photographed. Of slightly more relevance to the present invention is U.S. Pat. No. 4,783,269, which is directed to an injection control system for a flocculating agent which includes a flocculating pool into which the inflow liquid flocculating agent is injected and which forms flocs of suspended matters in the liquid; flocculating agent injecting means for injecting the flocculating agent into the flocculating pool; floc image pickup means for photographing a state of the flocs in the flocculating pool and for converting luminance data of the flocs into an electric signal; image recognizing means for recognizing the shape of the flocs by binarizing the image signal derived from the floc image pickup means on the basis of a luminance level of each pixel; flocculation state deciding means for calculating a characteristic amount of a diameter distribution of the flocs on the basis of the floc shapes recognized by the image recognizing means; and injection amount control means for controlling an amount of the flocculating agent which is injected from the flocculating agent injecting means on the basis of the characteristic amount.
However, the images generated and analyzed by the image recognizing means of the '269 patent are limited to shapes, sizes and luminance of the particles being photographed, essentially black and white photographs with shades of gray. To the eye of the image recognizing means of the '269 patent everything is a floc, the only questions are—what is its shape, how large is it, and what is its volume based on the luminance or intensity of the signal? Thus, the '269 patent fails to provide a system capable of recognizing differing species in a wastewater stream and adjusting the injection of a variety of chemical wastewater treatment agents in response.